Dual-cell high speed downlink packet access (DC-HSDPA) has been introduced in the Release 8 specifications of the third generation partnership project (3GPP) for universal mobile telecommunication systems (UMTS). With this feature, base stations (also referred to as Node-Bs) communicate to wireless transmit/receive units (WTRUs) over two distinct carriers simultaneously. It not only doubles the bandwidth and the peak data rate available to the WTRUs, it also has a potential to increase the network efficiency by means of fast scheduling and fast channel feedback over two downlink carriers.
FIG. 1 shows a medium access control (MAC) architecture for DC-HSDPA operations. The MAC layer architecture of DC-HSDPA includes one hybrid automatic repeat request (HARQ) entity per high speed downlink shared channel (HS-DSCH). This implies that HARQ retransmissions take place over the same transport channel and thus somewhat restricts the benefit of frequency diversity potentially brought by the use of more than one downlink carrier if each HS-DSCH has a fixed mapping to physical channel resources. However, it has been suggested that the mapping between the HS-DSCH and the physical resources (codes and carrier frequency) may be dynamically modified in order to provide a diversity benefit.